A reed relay consists of a switching device (such as the one described in U.S. Pat. No. 4,769,622), which can be a dry reed or mercury wetted switch, and an energizing coil for generating a magnetic field around the magnetic conducting parts of the switch and thereby generating a magnetic force for selectively opening and closing the switch. The coil is wound on a hollow tubular bobbin that defines a central aperture and is open at both ends, thereby allowing the switch to be introduced into the aperture of the bobbin. A thermoset material is then moulded around the coil-bobbin-switch assembly, or the assembly may be embedded in a potting compound such as polyurethane for fabricating the completed reed relay part.
During the moulding or embedding process, the thermoset material or potting compound flows through the bobbin's central aperture and directly contacts the switching device. Since the coefficient of thermal expansion for the thermoset material or the potting compound does not match the coefficient for the switching device (i.e., the coefficient of thermal expansion for the glass envelope that typically hermetically seals the conductive elements of the switching device), a change in temperature occurring at any time during the life span of the reed relay can cause thermal stresses that adversely affect the reed relay's performance. Such temperature changes and their resulting thermal stresses can occur during shipping, during installation (e.g., while soldering a reed relay onto a printed circuit board), or during operation of the reed relay occurring as a result of fluctuations in the ambient temperature. The thermal stresses resulting from such temperature changes can adversely affect the reed relay's operating characteristics such as its contact resistance (i.e., the electrical resistance between both ends of the switching device when closed) or its operate and release voltages (i.e., the voltages applied to the coils to open and close the switching device), and can also cause glass cracking, glass breakage, and failure of the reed relay.
One method of remedying some of these deficiencies in prior art reed relays is to condition the final relay with varying temperatures. Such methods attempt to bring the thermal characteristics of the thermoset material or the potting compound into equilibrium with the thermal characteristics of the switch. However, such methods are expensive since they add an additional step to the process of manufacturing a reed relay and they are also generally ineffective.
Another method of remedying some of these deficiencies in prior art reed relays is to mould a thermoplastic material rather than a thermoset material around the coil-bobbin-switch assembly and to select the thermoplastic material so that its temperature characteristics match those of the switching device. However, such relays can not operate over the same temperature range as relays produced using thermoset material or potting compounds.
Another problem with prior art reed relays is that the thermoset material, thermoplastic material, or potting compound does not flow evenly and predictably through the bobbin's central aperture and around the switching device. Rather than entirely encapsulating the switching device, the thermoset material, thermoplastic material, or potting compound tends to leave "voids" or unfilled regions around the external surface of the switching device. Such voids affect the operating characteristics of a reed relay, and since the voids tend to occur randomly in any given reed relay, it is difficult to produce a large quantity of reed relays that all provide the same operating characteristics.
Another problem with prior art reed relays is that it is difficult to entirely automate the process of manufacturing them, since the step of inserting the switching device into the central aperture of the bobbin must normally be performed manually.
Yet another problem with prior art reed relays is that the start and finish ends of the coiled-wire typically terminate on the bobbin terminals which are soldered or welded directly to the leadframe. A force applied on these bobbin terminals, which can occur during the assembling process or during the coil-to-leadframe welding process, can result in stressing the start and finish ends of the wire. This stressed wire is weakened and can break when additional stresses are generated by the external environment.